The present invention relates to an automatic focusing camera, and more particularly, to an automatic focusing camera of the lens interchangeable type which detects a difference in range between an image forming position and a predetermined focusing position of an object being photographed to drive a taking lens to the predetermined focusing position in response to the detected range difference.
In the past, a variety of automatic focusing cameras in which, when a shutter release button is softly half-depressed, an operation of detecting a range difference between an image forming position and a predetermined focusing position is initiated and a taking lens is driven to a focusing position in response to a detected output have been disclosed in Japanese Laid Open patent application Sho No. 56-1024 and others. With such an automatic focusing camera, a photographer directs it to an object being photographed and an automatic focusing operation is not initiated until a release button is intentionally half-depressed, so that the camera operation is simple and an automatic focusing operation is scarcely effected to an uninterested object.
In addition, most of such automatic focusing cameras of the lens interchangeable type employ a storage type photoelectric transducer such as a line image sensor employing a CCD (charge coupled device) as a detector for a range difference. The storage type photoelectric transducer can detect a range difference by light rays from an object being photographed which pass through a taking lens, so that it is suitable for mounting an automatic focusing apparatus on a single lens reflex camera of the lens interchangeable type.
However, the storage type photoelectric transducer, whose dynamic range to an amount of light is limited, is unsuitable under all photographable brightness conditions and therefore its storing time should be adjusted in accordance with the brightness of an object being photographed. As a result, the storing time for an object being photographed in a low brightness is increased and the time for detecting a range difference is consumed so much. Accordingly, in an automatic focusing camera of the conventional type in which the automatic focusing operation is not initiated until a release button is half-depressed, a time lag may be caused by hundreds (MS), when an object being photographed is in a low brightness, until a taking lens is set after a release button has been half depressed. The time lag is felt to be very long to a photographer to make him irritated.
On the other hand, with such an automatic focusing Camera, it is not to take a picture in a focused condition until a release button is half-depressed after an object being photographed has been positioned in a range finding frame centered in a view finder. Accordingly, a photographer is cautious in effecting the above stated operations, so that a time lag may be caused until a release button is half-depressed after a desired object to be photographed has been positioned within the range finding frame. The time lag will be greater than an estimated time and also a time required for detecting a range difference even with an object being photographed in a comparatively reduced brightness. Consequently, if the detection of a range difference can be repeatedly operated before a release button is half-depressed, it is possible to eliminate a time lag from half-depression of a release button to a lens focusing operation.